1. Field of the Invention
This invention relates to conductive compositions which are generally referred to as polymer thick films. More particularly, the invention relates to conductive compositions which demonstrate improved solderability and methods for their production.
2. Description of the Prior Art
The prior art discloses electrically conductive resinous compositions containing metal powders or flakes or combinations thereof with various resins, such as, for example, epoxy resins, phenolic resins and polyester resins.
U.S. Pat. No. 3,412,043 discloses an electrically conductive resinous composition consisting essentially of silver flake, resinous binder, and finely divided inert filler. The invention lies in the discovery that conductive cements and coatings can be prepared which have excellent electrical and physical properties by incorporating a substantial amount of inert filler.
U.S. Pat. No. 3,030,237 discloses an improved coating composition essentially comprised of an organic resin carrier and a suitable metallic pigment. The coating and the electrical equipment to which it is applied have improved adhesion and electrical operating characteristics in comparison with conventional conducting coating materials. In connection with printed circuit boards, conventional wire leads may be silver-soldered to the cured coating using a common silver-soldering technique.
U.S. Pat. No. 2,280,135 discloses a conductive coating composition comprising metal of the group consisting of nickel, tin, bismuth, cadmium, chromium and silver, in the form of finely divided flakes, dispersed in a liquid medium comprising a film-forming organic substance and a volatile solvent therefor. These coatings are opaque and are of low light reflectivity on glass and the like.
U.S. Pat. No. 4,353,816 discloses a power conductive coating consisting of a mixture in a liquid condition of 70-85 wt. % of copper powder, 15-30 wt. % of at least one selected from the group consisting of phenol resin, epoxy resin, polyester resin and xylene resin, and 0.2-5 wt. % of at least one annex agent selected from the group of anthracene, anthracene carbonylic acid, anthranylic acid and anthrazine.
The prior art compositions are deficient in that they are not readily solderable.
It has been a long sought goal to provide electrically conductive resinous compositions which are readily solderable and which would utilize available materials to achieve the desired purpose at minimal cost.
In order for a polymer matrix to be highly electrically conductive it must contain metal. However, a polymer matrix containing ordinary metal is not readily solderable. Those materials that claim solderability can only do so under closely controlled processing conditions; the temperature and time must be too closely controlled to be useful in a typical manufacturing environment. The use of precious metals such as gold and silver is an alternative; however, it can be well appreciated that because of the high cost of these materials this method is expensive. Present day polymer conductors typically do contain silver or gold but they are also not directly solderable using ordinary techniques.